The present invention relates to a semiconductor device such as a thin film transistor (TFT) or a thin film diode (TFD), or a thin film integrated circuit (IC) to which TFT or TFD is applied, and in particular, a thin film integrated circuit (IC) for an active-matrix addressed (active matrix) liquid crystal display device, having an insulated gate structure comprising a semiconductor film provided on an insulating substrate such as a glass substrate, or on an insulating coating formed on any type of substrate. The present invention also relates to a process for fabricating the same.
Semiconductor devices comprising TFTs on an insulating substrate (such as a glass substrate) developed heretofore include an active matrix-addressed liquid crystal display device using the TFTs for driving the matrices, an image sensor, and a three-dimensional IC.
The TFTs utilized in those devices generally employ a thin film silicon semiconductor. Thin film semiconductors can be roughly classified into two; one is a type comprising amorphous silicon semiconductor (a-Si), and the other is a type comprising crystalline silicon semiconductors. Amorphous silicon semiconductors are most prevailing, because they can be fabricated relatively easily by a vapor phase process at a low temperature, and because they can be readily obtained by mass production. The physical properties thereof, such as electric conductivity, however, are still inferior to those of a crystalline silicon semiconductor. Thus, to implement devices operating at an even higher speed, it has been keenly demanded to establish a process for fabricating TFTs comprising crystalline silicon semiconductors. Known crystalline semiconductors include polycrystalline silicon, microcrystalline silicon, amorphous silicon partly comprising crystalline components, and semiamorphous silicon which exhibits an intermediate state between crystalline silicon and amorphous silicon.
In case of fabricating an insulated gate structure using the silicon films enumerated above, an insulating film having excellent boundary characteristics must be fabricated by any means on the surface of the silicon film. For instance, a gate insulated film can be formed by thermal oxidation if a quartz substrate or any substrate resistant to high temperature is used. However, a quartz substrate is expensive, and is not suitable for large area substrates due to its too high a melting point. Accordingly, the use of other inexpensive glass materials (such as a Corning No.7059 glass) for the substrate is preferred because of its low melting point and its applicability to mass production. Those lower cost glass substrate materials, however, do not resist to a high temperature process for fabricating a thermal oxidation film. Thus, the insulating film is formed on those inexpensive glass substrates at lower temperatures by means of a physical vapor deposition (PVD) such as sputtering, or a chemical vapor deposition (CVD) such as plasma assisted CVD or photo CVD.
The insulating films formed by PVD or CVD process, however, suffer pinholes and inferior interface characteristics. Thus, the TFTs formed from these films yield problematic low field mobility and low sub-threshold characteristics (the S value), or a large leak current in gate electrode, a severe degradation, and a low production yield. In particular, these characteristics of a gate insulating film has not been found a problem in a TFT using
amorphous silicon having low mobility, however, in a TFT using a silicon film having high mobility, the characteristics of the gate insulating film are found more important than those of the silicon film itself.
An object of the present invention is to provide a means for solving the aforementioned problems, and to provide an oxide film on the semiconductor layer formed on an insulating substrate. In particular, the present invention provides a process for fabricating a gate insulating film as well as a structure of a gate insulating film using a crystalline silicon film for a TFT improved in device characteristics, reliability, and production yield, so long as the conditions do not affect the substrate materials.
Another object of the present invention is to provide, in addition to the active layer of a semiconductor device, a highly crystalline semiconductor layer.
The present invention is characterized in that it comprises forming a thin silicon oxide film on the surface of an island-like crystalline silicon film by irradiating an intense light to the semiconductor material (optically annealing) at a wavelength not influencing the substrate material under an oxidizing atmosphere such as of oxygen, nitrogen oxide, and ozone. Otherwise, the thin silicon oxide film (referred to hereinafter as xe2x80x9cthermal oxide filmxe2x80x9d, inclusive of the silicon oxide film obtained by irradiating an intense light) is formed by thermally annealing the island-like crystalline silicon film at a temperature not influencing the substrate material. In the present invention, the step of forming a thin silicon oxide film is followed by the step of forming a thick silicon oxide film covering the thin silicon oxide film by means of various types of known CVD processes to provide a gate insulating film of a desired thickness.
In particular, the silicon oxide film is obtained by subjecting an organic silane such as tetraethoxysilane (TEOS) as the silicon source together with an oxidizing agent such as oxygen, ozone, or nitrogen oxide, to a CVD process to form silicon oxide. The CVD process specifically refers to a reduced pressure CVD, a normal pressure CVD, a photo CVD, plasma CVD, and a combination thereof.
A silicon oxide film with still stable characteristics can be obtained by forming a silicon oxide film by CVD, and then photo annealing the silicon oxide film again using a visible light or a near infrared light or thermally annealing at a temperature in the range of from 400 to 700xc2x0 C., under an atmosphere of a gaseous compound of oxygen and nitrogen (e.g., N2O), or a mixed gas atmosphere (e.g., a 4:1 mixture of nitrogen and oxygen).
Furthermore, the inventors have discovered that a concentration of electron traps is undsirably high when the silicon oxide film is formed by an organic silane through CVD and the inventor considered that the traps are related to Sixe2x80x94OH bondings. In accordance with another aspect of the invention, the silicon oxide formed from an organic silane is annealed with a nitrogen containing atmosphere, for example, NH3, N2H4, N2 and N2O at a temperature from 400-850xc2x0 C., thereby, breaking the Sixe2x80x94OH bondings and improving the reliability of the gate insulated structure.
In the process according to the present invention, light is preferably irradiated for a relatively short duration of from about 10 to 1,000 seconds to elevate the surface temperature of the silicon film in the temperature range of from 900 to 1,200xc2x0 C. The light is irradiated to the silicon film, preferably, at a wavelength absorbed by the silicon film and substantially not absorbed by the substrate. More specifically, a light of a wavelength falling in the range for a near infrared region to the visible light region is preferred, and a light having a wavelength of from 0.5 xcexcm to 4 xcexcm (e.g., an infrared light having a peak at a wavelength of 1.3 xcexcm) is more preferred.
In the present invention described above, the thermal annealing is preferably effected at such a temperature that would not unfavorably influence the substrate as to form warping and shrinking thereon. More specifically, the thermal annealing is effected in the middle temperature range of from 400 to 700xc2x0 C., and more preferably, in the range of from 500 to 600xc2x0 C. In general, the thermal annealing is performed at a temperature not higher than the deformation temperature of the substrate, however, the strain energy accumulated inside the substrate can be released to sufficiently reduce the distortion by thermally treating the substrate prior to the thermal annealing. Accordingly, the thermal annealing in this case can be applied at a temperature not lower than the deformation temperature.
The crystalline silicon film for use in the present invention can be fabricated by using a laser or an intense light equivalent thereto to crystallize a non-crystalline silicon film, or by using a thermal annealing.
In accordance with another aspect of the invention, a thermal annealing at a temperature lower than the ordinary crystallization temperature for solid phase growth can be effected using nickel or another metal element. The elements for accelerating the crystallization include the Group VIII elements of the periodic table, i.e., iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt). Also included are the transition elements having the 3d electrons, i.e., scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), copper (Cu), and zinc (Zn). Further included are gold (Au) and silver (Ag) whose effect on the crystallization is experimentally observed. The most effective among the elements enumerated above is nickel, and the operation of a TFT based on the crystalline silicon film using the effect is verified.
A silicon film containing the elements above as an additive is observed to grow into acicular (needle-like) crystals. However, the crystal growth occurs not globally on the surface, and amorphous regions or regions of low crystallinity having a crystallinity equivalent to that of the amorphous region are left between the crystals. The silicon film containing any of the metal elements above grow into acicular crystals having a width of from 0.5 to 2 times the thickness of the film. Moreover, the crystal grows less along the width direction; i.e. in the direction toward the sides of the crystal, not along the crystallographic  less than 111 greater than  orientation. Accordingly, the deterioration in characteristics of the silicon film of this type has been found a serious problem when used in a TFT, because the amorphous region does not undergo crystallization even after an annealing effected for a long duration of time.
In the present invention, an intense light is irradiated to an amorphous silicon film. In such a case, however, a part of the optical energy is utilized for the crystal growth to accelerate the film growth along the direction toward the sides of the crystals. Accordingly, a dense crystalline silicon film can be obtained.
When using a ultraviolet light (UV) having a shorter wavelength than a visible light and the like, not only the silicon film but also the substrate material absorbs the light. Accordingly, the optimal duration of irradiating the light becomes shorter. For instance, the duration of irradiating a light 248 nm in wavelength is preferably not longer than 1 xcexcsec. If the light is irradiated for a still longer duration, the substrate undergoes deformation due to the light absorbed at an excessively large quantity. Thus, in case the light is irradiated for an extremely short period of time, the quantity of light must be selected as such which instantaneously elevates the surface of the silicon film to a temperature higher than 1,000xc2x0 C. Since the temperature is raised and lowered instantaneously, the oxidation can not proceed sufficiently by a single irradiation. Accordingly, the light must be irradiated for a plurality of times. The thickness of the oxide film which results by the repeated irradiation depends on the irradiation times.
In case irradiation is effected for a short period of time using UV as the light source, a laser operated in pulses, such as the excimer laser, is most preferred. An excimer laser has a pulse width of 100 nsec or less.
In the present invention, the substrate temperature may be elevated to a maximum of value of 600xc2x0 C., and preferably, to 400xc2x0 C.
The insulating film that is deposited by PVD or CVD after annealing the silicon substrate by either irradiating an intense light or annealing at a medium temperature is generally a silicon oxide film, however, it may be a silicon nitride film or a silicon oxynitride film. Furthermore, the irradiation of an intense light and the thermal annealing can be repeated for a plurality of times.
The thermal oxide film which is obtained by either irradiating an intense light or by annealing at a medium temperature is generally provided at a thickness of from 20 to 200 xc3x85, and representatively, at a thickness of 100 xc3x85. It differs from a known film obtained by PVD or CVD, and is an extremely dense film of a uniform thickness free from pinholes. Furthermore, it exhibits an ideal boundary with a silicon film. A thick insulating film, representatively, a silicon oxide film, is deposited on the thermal oxide film. Thus, a leak current due to pinholes is further prevented from occurring, and the production yield is still improved.
Furthermore, since a favorable interface is realized between the thermal oxide film and the silicon film, the use of the thermal oxide film further improves the characteristic values and the reliability of the TFT. In a prior art process for fabricating a TFT as illustrated in FIG. 4A, in particular, cavities tend to form on the edges of a silicon film upon fabricating an island-like silicon film due to over etching. This is found to occur most frequently in case a soft base film having a high etching rate is used. A prior art PVD or CVD could not bury the cavities, and leak current was found to generate frequently by the presence of cracks and the like (FIG. 4B).
In contrast to the prior art process described above, the formation of cracks is of practically no problem for the semiconductor device of the present invention because a thermal oxide film of a uniform thickness and free from defects such as pinholes is formed around the silicon film (FIG. 4C).
Previously, an oxide film of such a high quality was only obtained by a thermal oxidation at high temperatures. However, this requirement greatly limited the use of substrates. It can be seen that the present invention is free of such limitations concerning the heat resistance of the substrate.
Thus, the present invention provides a gate insulating film having a superior quality and reliability by forming a pinhole-free thin and dense thermal oxide film of uniform thickness on the surface of an active layer, either by irradiating an intense light at a wavelength not absorbed by the substrate to the island-like silicon film provided as the active layer of the TFT or by annealing the active layer in an oxidizing atmosphere at a temperature at which no warping or shrinkage may occur on the substrate, and further superposing, on the thin thermal oxide film, a thick silicon oxide film by CVD using TEOS and an oxidizing gas such as oxygen.
Conventionally, an oxide film of such a superior quality above has been obtained by thermal annealing at high temperatures. Accordingly, a great limitation had been posed on concerning the heat resistance. In the process and device according to the present invention, however, there is no limitation with respect to the heat resistance of the substrate material. Thus, various types of glass materials are applicable for the substrate, and great effect is found in case a material having a deformation temperature in the range of from 550 to 700xc2x0 C. is used. It can be seen that the present invention is greatly contributory to the industry.